It has been recognized for some time that patient controlled medicament delivery (PCDD) as in the case of patient controlled analgesia (PCA) is desirable in many situations. Before the advent of patient controlled medicament delivery, therapeutic treatments relied upon periodic injections of medicaments such as natural and synthetic opioids by a physician or nurse. This has the disadvantage that for most of the time the patient's medicament level may be significantly above or below the optimum.
PCDD improved on the prior art by enabling the infusion of small quantities of medicaments at regular intervals as perceived to be required by the patient. However, to date PCDD has been effected by sophisticated electronic pump systems, which have a number of disadvantages:                (a) They are expensive;        (b) They are complex and require skilled maintenance; and        (c) They are capable of administering an overdose as a result of machine failure or of operator error in setting up; a number of deaths from this cause have been reported.        
Recently mechanical PCDD pumping systems have been developed to ameliorate some of the disadvantages attendant with prior art devices. Such devices generally consists of a reservoir and a pumping assembly that contains a dose chamber which takes a predetermined amount of time to fill. These pumps have the disadvantage that filling of the dose chamber in the pumping assembly may take a long time and filling of the last portion of the dose chamber may be extremely slow. Moreover, if patients activate mechanical PCDD pumping systems prior to complete filling of the fluid dose chamber they may receive an excess of medicament. Thus, physicians may have no means of controlling the total amount of medicament delivered to a patient, leading to possible medicament overdosing by the patient.
Physicians generally associate the term “lockout” with a period of delay between medicament deliveries. They also have an expectation that the dose chamber in the delivery device will be 100% full at the end of each lockout period.
The filling cycle of electronic PCDD pumps is generally immediate. Electronic pumps allow a unit dose of medicament to be delivered and control a time interval where no further doses of medicament can be delivered. When this time interval is completed the patient can activate a switch which indicates his/her desire for another dose. The next unit dose will then be delivered and the next lockout will take effect.
In mechanical PCDD pumps the filling time of the dose chamber is progressive over a period of time that is equivalent to the predetermined lockout period. Typically, a concave filling curve is observed wherein the majority of the dose chamber fills rapidly after medicament delivery/release after which there is a slow and progressive filling of the last portion of the dose chamber. Often the filling time which leads to 100% filling of the dose chamber in such pumps is greater than the lockout period. Thus, a patient who activates the device prior to specified delivery times might obtain less than the absolute dose that is required to fill the dose chamber.
Depending on the type of PCDD pump employed, a patient may also gain significantly greater doses of a medicament than he/she should receive, by using the device at frequent intervals before the dose chamber is completely full. For example, a patient who activates a mechanical PCDD pump once every few minutes for an hour will gain significantly greater amounts of a medicament than they should receive if they use the pump once every 10 minutes over a 1 hour period. This is because the most rapid filling in mechanically controlled PCDD lockout pumps occurs in the first minutes. In some circumstances a patient may, for example, receive more than 200% of the expected dose of medicament if he/she activates the device at shorter time intervals than recommended for medicament delivery. This phenomenon has in the past led to patient overdose.
It has been found that by controlling the number of doses of a medicament that a patient receives per hour, it is possible to control many patient symptoms. In particular, patients can control their own symptoms by measuring the symptoms and adding doses of medicaments as required. In such situations physicians would choose the limit that will be an index of medicament safety for a certain dose to be delivered per hour.
The patient-controlled delivery device described in O'Neil et al (U.S. Ser. No. 08/973,591) provides a delivery device for patient-controlled infusion of a medicament, comprising a reservoir for the medicament and a pump having a predetermined delivery dose that is capable of displacing the medicament from the reservoir and delivering it to a patient. However, a problem that occurs with the O'Neil delivery device is that when the pump is tipped on its side, an air-fluid interface over the end of the suction tube may develop. This situation leads to air being sucked by the vacuum into the very fine bore tubing. The air within the tubing interferes with further functioning of the tubing. Thus, the presence of air in the tubing means that there is a reduction in the pressure gradient along the tubing and the resultant effect may be a failure of fluid to continue to flow along the tubing.